Creating a dome

Hi everyone. I am using BRepOffsetAPI_ThruSections to create nice 'loft'surface (as seen in the images), but how can I create a 'dome'? What I mean is - is there any way to close the shape at the end in a nice and smooth way? I tried to AddVertex to BRepOffsetAPI_ThruSections as a parameter, but is sharpens and the result is not what I want.

Thank you for your help in advance.